Parakinematyczna B-lineacja mezoskopowa ziarn mineralnych

Józef Oberc, Jerzy Kotowski


Mesoscopic parakinetmatic B-lineation of mineral grains

The parakinematie lineation of mineral grains occurs only in rooks subjected by to crystallization simultaneous with movements, i.e. chiefly in metamorphic rocks. Its presence can be also expected in sedimentary évaporite rocks which are prone to recrystallization. This type of lineation is absent in igneous rocks, with the exception of relic lineation inherited: from original metamorphic rodks. Parakinematie B-lineation is present on foliation surfaces and with laminae and/or beds, e.g. in granulites, thick-bedded limestones, and even in metasomatic quartz veins. Several mesoscopic types of B-lineation of mineral grains are presented in Fig. 1. Parakinematiic B-lineation of grains of several mineral species is present in the metamorphic rocks of the Sudetes Mts. Garnet: B-lineation marked by garnet is emphasized by linear arrangement of isolated grains. It occurs in granulites of the Sowie Góry Mts., in mica-schists of the metamorphic complex of the Izera Mts., in the Mt. Śnieżnik group, etc. Some of the garnets contain sigmoidally arranged inclusions. Amphiboles: the individual amphibole prisms represent linear forms displaying a definite orientation in space. The grains are arranged parallel, and occur in amphibolites, amphibole schists, and more rarely in other rock types, e.g. in dark-coloured varieties of the Izera Mts. gneisses. The lineation of amphiboles is often associated with lineation of biotite and chlorite. Diopside: the lineation, represented most often by rectilinear monomineral aggregates occurs in metamorphosed limestones and dolomites, as well as in lime silicate rocks. Quartz: parakinematie lineation of quartz occur in various morphological forms. One of these comprise elongated lenticular bodies (Fig. 5) of secretional quartz present in mica-schists and phyllites, and more rarely in other rock types. The lineation marked by quartz „fibres” (Fig. 6) is formed by strongly elongated quartz grains. This type of lineation is known from metamorphic quartz „veins”. Another type of quartz lineation is represented by quartz „dates” in „dattelquartzit” — type rocks, i.e. by elongated, flat, prolate grains, elongated concordantly with other B-lineations. Feldspars: elongation of prolate feldspar grains parallel to the b-coordinate of the structure occurs together with other lineations of mineral grains and aggregates (Fig. 7 and Fig. 8). The feldspar lineation is often rotated accordingly with the b-coordinate. Inclusions marking helieitie structures are present within grains showing such a lineation. The lineation of feldspars is present especially in gneisses in Lower Silesia. Tourmaline: tourmaline lineatidn is represented by monomineral aggregates of this component (Fig. 9). The long axes of tourmaline grains are parallel, and their parakinematie character is marked also by their arrangement parallel to other types of B-lineation, especially of biotite, quartz-feldspar aggregates and prolate feldspar grains. Tourmaline lineation is known from gneissie granites of the Izera Mts., and from the Kowary gneiss. Calcite: the lineation is marked by calcite „rolls”, with lenticular outlines in the ac plane. The rolls, several mm wide, occur on bedding; planes. They are related with intersecting shear planes, or, in other instances, with „boudinage”. Another example of calcite lineation is provided by elongated „threads” of this mineral, which are present in crystalline limestones. Minerals having lamellar or flaky form: biotite, chlorite, serieite, muscovite and graphite occur as single-component aggregates, within which the individual lamellae are displaced eoneordantly with foliation surfaces. Occasionally, especially in the case of biotite, the aggregates are felt-like, forming prisms or flat elongated lenses. Lineation of fibrolite has been found in the Strzelin gneisses. This mineral forms elongated specks, arranged parallely on foliation surfaces, which are concordant to other B-lineations.

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